The invention relates to an apparatus and a method for checking the authenticity of a security element, e.g. of the security element of a value document, as well as to a value-document processing apparatus having the apparatus for checking the authenticity of the security element.
In bank note processing systems properties of bank notes, such as e.g. printed image, face value, authenticity and soiling are ascertained, by physical properties of the bank notes being captured by means of sensors and the sensor data generated hereby being evaluated by means of algorithms.
For example, for checking the authenticity of security elements it is known to capture the transmission image of a security thread located on a bank note and to check the security thread for characters, so-called cleartext contained therein. Here, one makes use of the fact that in the transmission image the security thread usually appears dark or black, while at the places at which the gaps of the incorporated characters are located illumination light passes through and consequently the characters appear bright in transmission.
Additionally, there are also known security elements, e.g. security threads, which show to the viewer, when moving the security element, e.g. upon tilting the bank note, an optically variable effect, e.g. an optical movement effect or an enlargement effect. The optically variable effect of these security threads is generated, for example, by a microlens grid and a motif located therebehind, e.g. a grid-shaped pattern or characters disposed in grid-shaped fashion. They are designated as, e.g. security threads with “moiré effect” or with “modulo enlargement” or “motion threads”. The optical variable effect is provided for an authenticity check of the security element by the human viewer.
For checking such security threads by machine it is known from U.S. Pat. No. 8,965,099 B2 to examine with the help of an optical sensor whether the expected optically variable effect can actually be detected. For this, a bank note with a motion security thread is illuminated from at least two particular angles of incidence and from a suitable angle a remission image is recorded, accordingly two different motifs being detected. The two remission images are subtracted from each other and the difference values are added up. If—as in a forgery—from both angles the same pictures are detected, the difference values are zero and the sum accordingly is also zero. Only in the authentic motion security thread, the two remission images differ from each other, so that the calculated sum of the difference values exceeds a threshold. In this manner it is examined whether from the different angles there result different motifs in remission.
It is disadvantageous that in this method for different security threads having different microlens grids the necessary illumination/detection angles under which the motifs must be detected do vary. Hence, for different security threads different sensors are necessary. Additionally, for recognizing the motifs a high optical resolution of the sensor is required.